The molecular dynamics simulations have been employed to study the structural properties of 1-butyl-3-methylimidazolium tetrafluoroborate([BMIM][BF4])ionic liquids surrounding Au nanoparticles with different sizes.The simulation results reveal for the first time that the structural properties of imidazolium-based ionic liquids in Au nanoparticles depend on the size of nanoparticles at a molecular level.The structural properties are dominated by the alkyl chains of cations in the case of Au13 nanoparticle were found,while the imidazolium rings have the most contributions in the case of larger Au309nanoparticle.Meanwhile,the orientational distributions also show that cations prefer to locate on the Au309nanoparticles surface.Furthermore,the relevant interaction energies indicate that the interaction between the alkyl chain and the Au13nanoparticle much stronger than that from imidazolium rings,which is contrary to the behavior in the case of Au309 nanoparticle.In addition,all of the interactions between the anion and the Au nanopartciles is always the smallest regardless of the particle size,indicating that the anions have an indirect effect on the relevant structural properties.